1. Field of the Invention
The present invention is generally related to non-destructive inspection of pipes and tubing and in particular to positioning of inspection devices at various locations along the interior of serpentine-bent tube sections.
2. General Background
Many boilers have serpentine-bent horizontal tube sections. Such tube sections are generally of a 1 to 2 inch inner diameter with multiple (as many as 20) tight radius 180 degree bends and may be several hundred feet long. Non-destructive testing of the interior of the tubing is accomplished by the use of transducers. The type of mechanism used to move a transducer through a tube is chosen depending on tube diameter, the number, direction, extent, and radius of bends, and the length of the tube to be inspected. Moderately short lengths of tubing with few and gradual bends often depend upon a flexible cable similar to a plumber's "snake" for movement of an inspection transducer. Such devices are not known to be applicable to boiler tubes as described above. "Tractor" devices capable of negotiating the sharp bends and pulling a transducer and its signal cable along are also not known. Flexible cables pushed from the end of the tube are impractical beyond the first or second bend in the tubing due to the build up of drag forces. Various devices of which the inventor is aware include the following.
U.S. Pat. Nos. 3,583,393, 3,497,083, and 4,238,973 disclose selectively bendable tube assemblies and manipulators.
U.S. Pat. Nos. 2,748,803 and 4,648,733 disclose a reinforced hose and installation template for conduits.
U.S. Pat. Nos. 3,623,756 and 3,918,821 disclose articulated connectors.
U.S. Pat. Nos. 4,290,762, 3,020,362 and 3,197,954 disclose various link chains.
German patent no. 494,160 discloses a flexible shaft.
French patent no. 1,079,598 discloses a mechanical remote control that uses cables and springs inside a rigid conduit.
U.S. Pat. No. 4,896,555 discloses an inspection spine formed from identical links mated together by a pivot pin at bifurcated ends. Flat alignment faces are provided to prevent buckling during insertion. An oval bore for the pivot pin in one link allows pivoting of links at tubing bends.
Although there are a variety of flexible connectors available, there are few that fit the need to be able to travel through serpentine tube sections as described above where the device must be capable of being pushed through the tubes and then pulled back out once inspection is completed. Also, since there are a variety of tubes and tube configurations, such as tubes that have bends in more than one plane, no one system will serve all applications equally well.
The problems encountered are as follows. During insertion, when the device is being pushed through the tube, compression forces result in the links having a tendency to buckle at the link connecting pivot points, causing lock-up in straight sections of the tube and generating side loads that quickly become unacceptably high. Major drag forces are also generated in the bend areas of the tube during insertion and removal of known devices.